|Monthly Tech-Tip |
Refers to the quality of the dried ceramic glaze and how this affects the fired result: e.g. density, hardness, evenness, thickness, etc.
Key phrases linking here: glaze laydown, laydown - Learn more
In industrial ceramics, the quality and evenness of the dried glaze layer are considered very important in quality control and to the suitability of the final product. Industrial glaze application techniques and machines are highly tuned to apply an even and defect-free dried laydown. This is of paramount importance with many types of manufactured products where even a single pinhole can reject an entire item (for recycle, refire and discard).
Potters, on the other hand, think little about this. They just assume that since the glaze will be melting, surface irregularities will even out in the firing. Of course they do try to minimize drips, pinholes and cracks.
Laydown quality is best on porcelain, worst on heavily grogged bodies. Best on thin applications, and worse on thicks. Better with thixotropic slurries, and worse with inadequately suspended ones. Better when they dry quickly, and worse when they dry slowly. Better with glaze slurries having adequate fine particled clays in the recipe, worse with slurries low in clay content. Gummed slurries give the best quality laydown but require equipment and expertise to get a dense even layer that dries quickly. While it might seem that brushing glazes are potentially difficult to laydown evenly, commercial products have low specific gravities and must be applied in multiple layers, this enables achieving satisfactory laydown for hobby ware. Brushing likewise offers the advantage of making applications thicker or thinner on specific areas on an object. Spraying likewise has the potential for high quality laydown (in the hands of the experienced).
Dried glaze layer on a lightly grogged middle temperature stoneware (Plainsman M325). Notice how the bubbling that occurs during the drying of the glaze has disrupted the laydown. This is a glossy transparent and will likely level out, but if it were a matte or glaze having a stiffer melt, this texture would be evident in the fired piece.
Slow drying on porous, dry bisque is the cause. The one on the top left was dipped for three seconds, and the ones on the top and bottom rights for five seconds. Thus, the thicker the application the worse this is. This is a base-coat dipping glaze, it has enough gum to slow down drying significantly, providing plenty of time for escaping air, displaced by the water being absorbed into this coarser particled clay body, to create these holes. The bottom left? Since these tiles are thick this enabled quickly dipping it into water first to dampen the bisque, then into the glaze. That solved the problem.
This is a fluid melt cone 6 glaze with colorant added and partially opacified. It runs into contours during firing, thickening there (notice the darkening around the logo), this is a desired visual effect. However, notice that drips and runs coming down from the rim, they are producing darker streaks. This is an application issue. Glazes that fasten-in-place too slowly will drain unevenly on extraction from the bucket (after dipping). This can be solved by making a thixotropic slurry. If bisque ware is too dense, glazes have a more difficult time fixing-in-place in an even layer, especially if they have no thixotropy. If glazes lack clay (e.g. less than 15% kaolin) they do not gel as easily. Slurries containing too much gum dry slowly and drips are almost unavoidable. If the problem is too much melt fluidity, choose a more stable base glaze can really help. Just because melt fluidity is less does not mean that it will be less glossy.
This stoneware jar is made from a large particle size body with grog added. It was engobed on the inside and over the lip at the leather hard stage and bisque fired. Now it has been dip-glazed inside and out. Because the clay is so porous, lots of air must escape from inside the wall as the matrix soaks up water from the freshly applied glaze. But the air is being channeled into pathways and concentrated into surface irregularities created by the coarse particles in the matrix. At each of the escape sites a bubble appeared (then healed into a depression when it burst). But the engobe on the inside creates a homogeneous surface that distributes air escape uniformly, it does not have a single blister.
On a clay test tile this titanium opacified cone 6 oxidation glaze, G1214Z1, looks great. But it is important to recognize that its variegated fired appearance is a combination multiple factors: The chemistry of the glaze, the titanium, quality laydown, the PLC6DS firing schedule, the red M390 clay body and variations in the thickness-of-application. However, the last four of those factors changed with the mug on the right! It is made from buff-burning M340! There are drips from uneven drainage during glazing. The slow cool C6DHSC firing schedule. Notice how it is actually going transparent where very thick. An even laydown was not achieved since the slurry was not properly mixed, it contains calcined kaolin and requires special attention to achieve a thixotropy.
|By Tony Hansen|
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